The present invention relates to a method and apparatus for determination of an equipment failure or pilferage in a tank storage system having the ability to detect such failure or pilferage in a short period of time. More particularly, the present invention relates to a precision tank monitoring system and inventory reconciliation algorithm which accounts for all relevant variables, including dimensional variances in the tank, calibration of output metering systems, and delivery shortages or overages.
In a tank storage facility it is important to determine whether all materials can be accounted for. The United States Environmental Protection Agency (E.P.A.) requires the owner of a storage facility to account for all material in order to insure that there is no loss of material to the environment. The requirements stated by the E.P.A. include a tank precision tightness test or a monitoring system which has the capability to find small leaks. For an underground storage facility containing fuel such as gasoline, diesel, or jet fuel, the requirements include that either a precision test be run at least every five years (a precision test system must be certified capable of finding a 0.1 gallon per hour (g.p.h.) leak), or the tank be fitted with a monitoring system capable of detecting a 0.2 g.p.h. leak. In addition the inventory must be reconciled to a limit not greater than 1% of throughput plus 130 gallons on a monthly basis. If the inventory cannot be reconciled, a "show cause" rule requires that the tank owner show why the tank system is out of compliance. In addition if the failure is due to a loss of material into the environment, the tank owner is liable for the remediation of the problem, and may be subject to fines levied by the regulatory agencies. It is evident that the operator or owner of a tank storage facility needs a system which can reconcile the inventory and provide an early warning of an equipment failure which leads to a loss of material.
The equipment technology of the past has allowed owners to choose among systems which are capable of detecting a material loss to the environment and inventory reconciliation systems, including statistical inventory reconciliation, or automatic tank gauging systems. One method used to measure for a material loss to the environment is to place liquid or vapor detectors around the equipment as shown in U.S. Pat. Nos. 4,740,777 and 4,646,069. In case of a loss, it is evident that there must be contamination of the environment before an alarm state is indicated by such equipment. In addition, this method requires a separate inventory reconciliation system.
Statistical inventory analysis services as currently practiced by one vendor requires the tank operator to take daily recordings of the fluid height in each tank to an accuracy of 1/8 inch and supply this information, along with the total amount of fluid dispensed from the tank, and the total amount of fluid added to the tanks to a vendor who then reconciles the data each month. After a period of one or more continuous months of gathering this data the vendor of the service professes to be able to statistically determine a loss rate if that loss rate exceeds 0.2 g.p.h. The limitation to this system is that a loss to the environment may continue for a period of one or more months before detection. In practice this period may be shorter if the loss rate is higher, however, at higher loss rates, other detection methods may react first.
A second method which is currently practiced uses permanently mounted probes in the tank to provide an inventory reconciliation tool which is alleged to be capable of running a precision tank tightness test as well as to provide a continuous indication of gross volume of material in the tank along with a net volume corrected to a specified temperature. These systems must be supplied with a table (called a strapping chart) which indicates the volume of the tank versus the height or depth of material in the tank. In addition, these systems require that a coefficient of expansion be supplied in order to correct the gross volume to a net volume of material at a standard temperature. It has been noted by the vendors of this equipment that the table of height versus volume for a particular tank provided by the tank manufacturer is not accurate enough to allow inventory reconciliation. In an attempt to correct this problem, at least one vendor has patented a system to correct this table by assuming that the reconciliation data from the sensors and the delivery meter system is correct and calculating the table from this data (see U.S. Pat. No. 4,977,528). This patent discloses that the table is created by a process of iteration until further iterations reduce the accuracy of the table. It should be noted that these systems reconcile inventory on a monthly basis, although it is possible for these systems to detect a leak in the tank with a shorter delay.
It has been noted that a system which can reconcile data on a daily basis would be desirable if the equipment were available. In addition, the ability to determine the source of any failure in reconciliation would be desirable. Known systems have not been able to provide this capability.